Vehicle alternator output rectifier instrument capable of high temperature operation

ABSTRACT

A vehicle alternator output rectifier instrument in which mesa structure Zener diodes, formed by diffusion of impurity on a P-type silicon substrate, are used as rectifier elements which are used in rectifier circuits for rectifying an alternate current output of a vehicle alternator. Since the vehicle alternator output rectifier instrument employs the above Zener diodes, this rectifier instrument can endure high temperature circumstances, considerably reduce a reverse leakage current to thereby prevent a battery voltage from being discharged and reduce a radio noise to thereby reduce a bad influence exerted upon an electronic device serving as a load.

BACKGROUND OF THE INVENTION

The present invention generally relates to rectifier instruments forrectifying an output generated from an alternator for vehicles such as acar or the like and, more particularly, is directed to a vehiclealternator output rectifier instrument suitable for suppressing anover-voltage and for reducing a radio noise.

Recently, the kind of and the number of electronic devices that aremounted on vehicles as electrical loads have increased so remarkablythat a malfunction of these electronic devices becomes a problem. Toavoid the malfunction of the electronic devices, a DC (direct current)voltage that is obtained from a rectifier when the rectifier rectifiesan output of an alternator driven by a car engine, must be preventedfrom generating an over-voltage.

In order to protect the electronic devices from the over-voltage, aconventional vehicle alternator output rectifier instrument employs asrectifier elements Zener diodes which cause a breakdown at a relativelylow voltage as is disclosed in Japanese Published Patent PublicationNos. JP-2B-54-5083 and JP-A-63-240336.

When the Zener diodes are mounted on the car, the following pointsshould be considered.

(1) Because the vehicle alternator output rectifier instrument ismounted in the engine room of the car, it is exposed to high temperatureand utilized under severe temperature circumstances.

(2) Because the vehicle alternator output rectifier instrument isconstantly connected to a car battery even when the car is not driven, areverse leakage current is generated through the rectifier elementswithin the rectifier instrument, resulting in the car battery beingdischarged.

(3) A radio noise generated from the vehicle alternator output rectifierinstrument has to be suppressed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a vehicle alternatoroutput rectifier instrument using Zener diodes which can be adapted tohigh temperature circumstances like the engine room of the car.

Another object of the present invention is to provide a vehiclealternator output rectifier instrument which can be adapted to hightemperature circumstances like the engine room of the car and which canreduce a reverse leakage current and a radio noise.

According to an aspect of the present invention, in order that therectifier instrument can be adapted to high temperature circumstances,mesa structure Zener diodes that can realize heat radiation effect byheat conduction are employed as rectifier elements used in the rectifierinstrument.

According to another aspect of the present invention, by using P-typesubstrate Zener diodes, a satisfactory Zener characteristic can beobtained to reduce a reverse leakage current. Also, since the P-typesubstrate Zener diodes have a small resistivity, a reverse recovery timecan be reduced to thereby make it possible to reduce a radio noise.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of other objects, features, and advantages of thepresent invention can be gained from a consideration of the followingdetailed description of illustrative embodiments thereof, in conjunctionwith the figures of the accompanying drawings, in which:

FIG. 1 is a cross-sectional view showing a vehicle alternator accordingto the present invention;

FIG. 2 is an electric circuit diagram showing an example of a circuitprovided on the output side of the vehicle alternator shown in FIG. 1;

FIG. 3 is a front view showing a cooling plate to which the Zener diodesshown in FIG. 1 are attached;

FIG. 4 is a rear view of the cooling plate shown in FIG. 3;

FIG. 5 is a circuit diagram showing a controller 23 of FIG. 2 more indetail;

FIG. 6 through 9B are cross-sectional views showing structures of Zenerdiodes according to the present invention, respectively;

FIGS. 10 and 11 are diagrams showing structures of the prior art,respectively;

FIG. 12 is a diagram showing characteristic curves obtained when acharacteristic of the inventive diode and characteristics of theprior-art diode are compared and, to which references will be made inexplaining the present invention;

FIG. 13 is a table showing compared results of the inventive Zener diodeand the conventional avalanche diode, and to which references will bemade in explaining the present invention;

FIG. 14 is a graph showing a waveform of a surge current obtained whenthe avalanche diode is used;

FIG. 15 is a graph showing a surge characteristic of a rectifier circuitthat employs the inventive Zener diode;

FIG. 16 is a diagram showing a reverse recovery time obtained when theconventional avalanche diode is used;

FIG. 17 is a diagram showing a reverse recovery time obtained when theinventive Zener diode is used; and

FIG. 18 is a diagram showing technical effects achieved by the presentinvention in the form of a table.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinafter be described in detail withreference to the drawings.

FIG. 1 of the accompanying drawings shows a cross-sectional view of avehicle alternator. Referring to FIG. 1, a turning force of a car engineis transmitted through a belt (not shown) to a pulley 30 to rotate ashaft 31 which is supported by bearings 32 so as to become freelyrotatable. An exciting winding 10 is attached to a rotor 36 which isrotated in unison with the above shaft 31. A magnetic field generatedfrom the exciting winding 10 and an armature winding 11 on the statorside of the vehicle alternator are interlinked to generate analternating current (AC) in the armature winding 11. A DC (directcurrent) output current from a rectifier circuit, which will be belowdescribed, is supplied to the exciting winding 10 through slip rings 34.

FIG. 2 of the accompanying drawings is an electric circuit diagramshowing an overall circuit arrangement of the output side of the vehiclealternator shown in FIG. 1. As shown in FIG. 2, there is provided aone-dot chain line block 1 which includes a three-phase alternator and arectifier circuit for rectifying an output of the three-phasealternator. Armature windings 11u, 11v and 11w are connected in aY-connection fashion. Output ends of the respective armature windings11u, 11v and 11w and a neutral point 11n among the armature windings11u, 11v and 11w are respectively connected to Zener diodes 12 through19 so as to constitute a full wave rectifier circuit. An output terminalof the rectifier circuit 1 is connected to the exciting winding 10 andthen is connected to a succeeding circuit which will be described lateron.

The Zener diodes 12 through 19 are connected to the crescent-shapedcooling plate 35 (see also FIG. 1) shown in FIGS. 3 and 4 of theaccompanying drawings. FIG. 3 shows a front surface of the cooling plate35 and FIG. 4 shows a rear surface thereof. The cooling plate 35 ismounted within a housing of the alternator shown in FIG. 1. Cooling fans33 are attached to both sides of the rotor 36, and a cooling windgenerated when the cooling fans 33 are rotated cools the cooling plate35 to radiate heat from the Zener diodes 12 through 19.

In FIGS. 3 and 4, reference numerals 37 and 38 depict conventionalconnecting portions which connect the Zener diodes 12 to 19 and thearmature windings 11u, 11v and 11w.

Referring back to FIG. 2, there is provided a semiconductor voltagecontrol apparatus which is shown by a one-dot chain line block 2 in FIG.2. As shown in FIG. 2, the semiconductor voltage control apparatus 2includes a power transistor 21 for controlling an exciting current ofthe exciting winding 10, a power transistor 22 for controlling a chargewarning lamp 3 that indicates the charged condition of a battery 5, acontroller 23 for controlling the above power transistors 21 and 22, anda fly-wheel diode 24. In FIG. 2, reference numeral 4 depicts a keyswitch for starting the car, reference numerals 6, 6' depict a pluralityof electrical loads and reference numerals 7, 7' depict load switchesfor these electrical loads 6, 6'.

FIG. 5 of the accompanying drawings is a circuit diagram showing thecontroller 23 in FIG. 2 more fully. In FIG. 5, reference symbols athrough f depict terminals which are connected in association withreference symbols a through f shown in FIG. 2.

Referring to FIG. 5, an output voltage Vb (see FIG. 2) from therectifier circuit 1 is supplied through a terminal b to a series circuitof a resistor R1 and a Zener diode 41, thereby a constant voltage beinggenerated across the Zener diode 41. A voltage of the battery 5 issupplied through a terminal c to a series circuit of voltage-dividingresistors R2 and R3. A divided voltage from the voltage-dividingresistors R2, R3 is supplied to a negative input terminal of acomparator 42. The constant voltage developed across the Zener diode 41is divided by voltage-dividing resistors R4 and R5, and a dividedvoltage is supplied to a positive input terminal of the comparator 42.When the voltage of the battery 5 goes to high level, the voltage inputto the negative input terminal of the comparator 42 becomes higher thanthe voltage input to the positive input terminal thereof so that thecomparator 42 produces a low level output. This low level output issupplied through the terminal f to the power transistor 21 (see FIG. 2)and the power transistor 21 is therefore turned off, resulting in thefield current of the exciting winding 10 being decreased. Conversely,when the voltage of the battery 5 goes to low level, the comparator 42produces a high level output and the power transistor 21 is thereforeturned on, resulting in the field current of the exciting winding 10being increased. The controller 23 repeats the above operation tocontrol the battery 5 so that the voltage of the battery 5 is held atthe constant value.

A half-wave rectifying voltage from the rectifier circuit 1 is suppliedto the terminal a. This voltage applied to the terminal a is suppliedthrough a resistor R6 to a diode 43 and a capacitor 44, in which it isfiltered and then fed to a negative input terminal of a comparator 45.The comparator 45 is supplied at its positive input terminal with adivided voltage which results from dividing the constant voltagedeveloped across the Zener diode 41 by voltage-dividing resistors R7,R8. When the alternator is disabled under the condition that the keyswitch 4 is turned on, the voltage is not supplied to the terminal a sothat the output of the comparator 45 goes to high level to turn thepower transistor 22 on, thereby energizing the charge warning lamp 3 torepresent that the battery 5 is not charged normally. When on the otherhand the alternator is driven, the output of the comparator 45 goes tolow level to turn the power transistor 22 off, thereby disabling thecharge warning lamp 3.

According to the above circuit arrangement, if an output line of thealternator having a voltage Vb is disconnected when the alternator isrotated at high speed, a battery load dump surge as shown in FIG. 14will occur on the output line of the alternator. As a result, the outputof the comparator 42 goes to low level to turn the power transistor 21off, the exciting current from the exciting winding 10 is kept beingflowed through the fly-wheel diode 24. In that case, if ordinary diodesare used as the rectifier cells of the rectifier circuit 1, then theoutput voltage Vb of the rectifier circuit 1 rapidly rises temporarily.However, according to this embodiment, Zener diodes are used as therectifier elements so that the output voltage Vb of the rectifiercircuit 1 is clamped by a Zener voltage. Therefore, the surge voltagecan be prevented from being applied to the semiconductor voltage controlapparatus 2 and the electrical loads 6, 6'.

FIG. 6 of the accompanying drawings is a cross-sectional view showing astructure of a Zener diode that is used in the inventive rectifierinstrument as the rectifier element. In FIG. 6, reference numeral 50depicts a Zener diode chip. This Zener diode chip 50 has triple layersfabricated by diffusion of N-type impurity and P-type impurity on aP-type substrate and also has a mesa structure as shown in FIG. 7A ofthe accompanying drawings. An N⁺ side of the Zener diode chip 50 isinterconnected to a lead 53 by means of a solder 51, and a P⁺ sidethereof is interconnected to a disk 54 by means of a solder 52. The disk54 is connected to a metal case 56 by means of a solder 55 and the metalcasing 56 is connected to the cooling plate 35 (see FIG. 1). The disk 54is made of a material such as copper (Cu), molybdenum (Mo) or the like.The disk 54 plays a role of a heat sink having a heat capacitysufficient so that heat generated by a large current is flowed to theZener diode chip 50 is temporarily accumulated and the heat thusaccumulated is radiated with time. These chip structure elements arefabricated into the metal case 56 with a sealant 57 made of a materialsuch as silicon or the like, on which a passivation is effected. TheZener diode chip 50 shown in FIG. 7A forms a negative element as shownin FIG. 7B of the accompanying drawings. FIG. 8 of the accompanyingdrawings shows the case such that the Zener diode chip 50 is formed as apositive element. The structure of this Zener diode chip 50 shown inFIG. 8 is illustrated in FIGS. 9A and 9B of the accompanying drawings.From FIGS. 7A and 9A, it can be understood that the positive element andthe negative element are different only in polarity and that they havethe same triple-layer structure. Accordingly, the positive element andthe negative element can be fabricated via the same process.

Advantages brought about when the Zener diode chip of the presentinvention is formed as the mesa structure will be described below. Whenthe Zener diode chip is formed as the mesa structure, as shown in FIGS.6 and 8, the Zener diode chip 50 can be interconnected to the lead 53 ordisk 54 by soldering the whole surface of the respective upper and lowersurfaces. Therefore, the respective surfaces of the Zener diode chip 50can be wholly utilized to radiate heat. Hence, the Zener diode chip 50can be cooled effectively. In addition, since the disk 54 is used as theheat sink, a cooling efficiency can be improved much more. If on theother hand a chip having a planar structure is utilized, as shown inFIG. 10 of the accompanying drawings, although one surface (P-type side)of upper and lower two surfaces of the chip 50 can be wholly soldered,the other surface (N-type side) thereof must be soldered within alimited range of area. Consequently, a sufficient heat radiation areacannot be obtained. Therefore, such Zener diode chip having a planarstructure cannot be utilized substantially under severe temperaturecircumstances in actual practice.

The mesa structure has a uniform shape of its junction plane as comparedwith the planar structure and has a better Zener characteristicaccordingly. Therefore, the mesa structure has a small reverse leakagecurrent.

According to the present invention, there is provided the Zener diodewhose chip is formed by diffusion of impurity on the P-type substrate.This Zener diode will hereinafter be referred to as a Zener diode formedfrom the P-type substrate. Therefore, in the layer structure within theZener diode chip, as shown in FIGS. 7A and 9A, a distance between ajunction portion 47 and an edge portion 48 is relatively large. There isthen the small probability that a Zener characteristic will be affectedeven when the edge portion 48 that tends to be damaged mechanically isdamaged. Thus, the Zener diode of the present invention can obtain asatisfactory Zener characteristic as an electric power Zener diode.Hence, the Zener diode of this invention can considerably reduce thereverse leakage current that discharges the charged voltage of thebattery.

FIG. 11 of the accompanying drawing shows a structure of a Zener diodewhose chip is formed by diffusion of impurity on the N-type substrate.As shown in FIG. 11, the distance between the junction portion 47 andthe edge portion 48 is reduced comparatively. Therefore, when the edgeportion 48 on which a mechanical stress tends to be concentrated isdamaged, a Zener characteristic is unavoidably deteriorated. Morespecifically, having compared a normal Zener characteristic representedby a solid curve 3 in FIG. 12 of the accompanying drawings with otherZener characteristics, it will be appreciated that the Zener diode chiphaving the damaged edge portion causes a breakdown at a low voltage asshown by a two-dot chain curve 1 or that the above chip tends togenerate a leakage current as shown by a one-dot chain curve 2.

When the P-type substrate is used, if both of the P-type substrate andthe N-type substrate are the same in resistivity, then a life time ofcarrier can be reduced by half as compared with the case that the N-typesubstrate is used. Therefore, a life time of carrier within the chip isreduced with the result that the reverse recovery time is reduced.

When the conventional Zener diodes are used as the rectifier cells, theover-voltage can be suppressed. When, however, the Zener diodes in thethree-phase full rectifier circuit are at commutation, two-phase Zenerdiodes in the three-phase Zener diodes are simultaneously conducted anda short-circuit takes place between the output ends of the two-phasearmature windings, resulting in a large current in the rectifiercircuit. This large current is a surge current when the Zener diodes areat commutation. A higher harmonic component of the surge current issupplied to the output side of the rectifier circuit and acts as a radionoise to exert a bad influence on electronic devices serving as loads.The surge current generated when the Zener diodes are at commutation isincreased more if the reverse recovery time of the Zener diode, i.e, aperiod of time until the reverse current is progressively attenuated tozero after a voltage applied to the Zener diode has changed from thepositive direction to the reverse direction, becomes longer. That is,the longer the reverse recovery time becomes, the surge currentgenerated when the Zener diodes are at commutation is increased muchmore. Therefore, a radio noise level is increased. Since the inventiveZener diode can reduce the reverse recovery time as mentioned above, thesurge current generated when the Zener diodes are at commutation can bereduced and the radio noise level can be lowered accordingly.

The inventive Zener diode has the junction portion formed by diffusionof impurity on the P-type wafer and therefore can be fabricatedinexpensively as compared with an epitaxial type Zener diode because theepitaxial type Zener diode needs a relatively long time to grow crystaland becomes expensive. If a life time killer such as platinum (Pt) orgold (Au) injected in order to reduce the reverse recovery time is used,then a material cost and a process cost will be increased much more.Since, however, the inventive Zener diode has the short reverse recoverytime as earlier noted, the life time killer is not required. Therefore,the inventive Zener diode can be fabricated inexpensively and becomeseconomical.

As rectifier cells used in the rectifier circuit that rectifies theoutput of the vehicle alternator, there are generally utilized avalanchediodes according to the prior art. The avalanche diode and the inventiveZener diode will hereinafter be compared to each other with reference toFIG. 13 of the accompanying drawings. The withstand voltage will bedescribed first. While the avalanche diode has an avalanche breakdownvoltage of about 300 Volts, the inventive Zener diode has an avalanchebreakdown voltage of about 30 Volts, i.e., substantially one-tenth ofthe voltage of the avalanche diode. Therefore, according to theinventive Zener diode, it can be avoided that the over-voltage generatedat the output side of the rectifier circuit exerts a bad influence uponthe electronic device serving as the load. A resistivity within thecrystal will be described next. A resistivity of the inventive Zenerdiode is about 1/200 of that of the avalanche diode and an impurityconcentration also can be increased, whereby the reverse recovery timecan be reduced to about one-fourth. Thus, the inventive Zener diode caneffectively prevent the over-voltage from affecting the load and reducethe radio noise that affects the load.

When the above avalanche diodes are used in the rectifier circuit, if apower supply path to the whole of or part of the electronic deviceserving as the load is cut by a disconnection or connection failureunder the condition such that a revolution rate of the alternator is10000 r.p.m., then as shown in FIG. 14 of the accompanying drawings, theoutput voltage of the armature winding rapidly and temporarily rises upto about 140 Volts at first and is then progressively lowered with timedue to the fly-wheel diode 24 (see FIG. 2). According to the presentinvention, as shown in FIG. 15 of the accompanying drawings, the rise ofthe output voltage of the armature winding is restricted at about 30Volts owing to the Zener characteristic and then progressively decreasedowing to the work of the fly-wheel diode 24 after about 100 millisecondshave passed. Then, by the action of the controller 23, the outputvoltage of the armature winding is held at about 15 Volts. Hence, thepresent invention can protect the electronic circuit from theover-voltage generated from the alternator.

The aforesaid reverse recovery time will described. The avalanche diodehas a relatively large reverse recovery time τ as shown in FIG. 16 ofthe accompanying drawings. Accordingly, the surge current corresponds toa hatched area shown by reference symbol A in FIG. 16. According to thepresent invention, as shown in FIG. 17 of the accompanying drawings, thereverse recovery time τ is small and the surge current becomesconsiderably small as represented by a hatched area B.

The foregoing description will be concluded below with reference to FIG.18 of the accompanying drawings. As mentioned before, the Zener diodehaving a mesa structure chip is superior to the Zener diode having aplanar structure chip from a rectifier cell cooling effect standpoint.The P-type substrate Zener diode is superior to the N-type substrateZener diode from a Zener characteristic standpoint, i.e., reverseleakage current standpoint. Study of comparison of the Zener diodehaving a mesa structure chip with the Zener diode having a planarstructure chip reveals, in particular, that the planar structure chip issuperior to the mesa structure chip because the planar structure chip isfree from the damage of the edge portion and that the planar structurechip is inferior to the mesa structure chip from a Zener characteristicstandpoint because the shape of the junction plane of the planarstructure chip is not uniform as compared with that of the mesastructure chip. FIG. 18 illustrates the above compared results in theform of a table wherein open circles represent "superior", solid "X's"represent "inferior" and open triangles represent "neither superior norinferior", respectively. Study of the table of FIG. 18 reveals that themost excellent Zener diode structure is the Zener diode having the mesastructure in which the P-type substrate is employed.

While the inventive Zener diodes are applied to all of the rectifiercells as described above with reference to FIG. 2, the present inventionis not limited thereto. If the Zener diode of these Zener diodes 12through 19 is applied to at least one rectifier cell, then it ispossible to achieve the same effects of the present invention. It is, ofcourse, desirable that the inventive Zener diodes are applied to allrectifier cells.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments and that various changes andmodifications could be effected therein by one skilled in the artwithout departing from the spirit or scope of the invention as definedin the appended claims.

What is claimed is:
 1. A vehicle alternator output rectifier instrumentcomprising:(a) an alternator for generating power by utilizing a turningforce obtained from a vehicle. (b) a rectifier circuit comprising aplurality of rectifier elements coupled to the alternator for convertingan alternate current output of said alternator to a direct current; and(c) a voltage control apparatus coupled to the rectifier circuit forcontrolling a direct current output voltage from said rectifier circuit,wherein:at least one of said rectifier elements of said rectifiercircuit is a Zener diode having a Zener breakdown voltage in a reversebiased direction thereof, whereby an output of said rectifier circuit islimited to a maximum voltage equal to said Zener voltage; said Zenerdiode has a mesa structure; and said Zener diode is thermally coupled toa heat sink connected to a metal case of said Zener diode, fortemporarily accumulating heat generated by a temporary large currentflowing through said Zener diode.
 2. The vehicle alternator outputrectifier instrument according to claim 1, wherein said rectifierelement includes a chip fabricated by diffusion of impurity on a P-typesubstrate.
 3. The vehicle alternator output rectifier instrumentaccording to claim 1, wherein said rectifier element includes a chipfabricated without injection of a life time killer.
 4. The vehiclealternator output rectifier instrument according to claim 2, whereinsaid chip is fabricated without injection of a life time killer.